The present invention relates to commercial vehicle suspensions and, more particularly, to a dual-stage tapered leaf spring for use in a tractor-trailer suspension assembly.
In general, most trailers, (such as specialty carriers, tankers, dry freight haulers, etc.) are equipped with non-driven single, tandem or multi-axle assemblies. Conventionally, the suspension systems provided for supporting and damping the relative movement between each axle and the trailer frame have included single-stage multi-leaf springs, pneumatic spring systems or a combination thereof. The vast majority of trailers are equipped with single-stage multi-leaf springs which are designed to mechanically dampen the trailer when "loaded" to preserve its cargo and provide adequate roll stiffness. Multi-leaf springs are a class of leaf springs having a plurality of three or more constant thickness stepped-length leafs which are stacked to form a constant rate leaf spring assembly. As such, single-stage multi-leaf springs are not designed to differentiate between "loaded" and "unloaded" trailer operation and thus normally provide a firm or "stiff" ride during loaded operation. Unfortunately, this "stiff" ride causes excessive suspension vibration and reduced wheel control during "unloaded" trailer operation which detrimentally impacts the useful service life of the trailer while causing an undesirably harsh ride for the vehicle operator.
Conventionally, trailer suspension applications equipped with dual-stage leaf springs for providing a variable or progressive rate (i.e. "soft" ride when unloaded and "stiff" ride when loaded) have been extremely limited due to the availability of pneumatic systems. However, when dual-stage leaf springs have been used it is common to employ a massive and in-efficient first stage multi-leaf spring having an additional second stage leaf mounted thereto. Traditionally, the first stage multi-leaf spring is sized to provide the low rate "soft" ride when the trailer is unloaded (i.e. curb load) with the second stage leaf being inactive. When the trailer is loaded (i.e. design load), the second stage leaf becomes actively loaded for causing the overall rate to increase so as to produce a firmer ride. According to one method, a "helper" spring is mounted above the main spring of the first-stage multi-leaf spring and does not support any load until it engages camming pads for resisting further deflection of the multi-leaf first stage. As such, the change in rate and, in turn, the ride stiffness is necessarily abrupt and harsh. Alternatively, dual-stage leaf springs may have one or more relatively thick second stages leafs mounted below and adjacent to the shortest leaf of the "first stage" portion of the multi-leaf leaf spring. Upon deflection, rolling contact is made between the second stage leafs and the first stage for producing the increased rate. Again however, the rate transition is typically abrupt.
While dual-stage leaf springs have been used in various light-duty truck applications, such springs have not been used in the heavy-duty trailer industry. This is primarily due to the fact that heavy-duty trailer suspensions must be designed to function for a significantly larger load-carrying range than is required of modern ligh-duty vehicles. As such, dual-stage multi-leaf springs are heavy and require a significant range of deflection to provide the desired rate transition. These design constraints have made utilization of conventional multi-leaf dual-stage springs impractical for many trailer suspension applications.
Modernly, pneumatic suspension systems are being installed in trailers to provide means for variably adjusting the rate in response to changes in the load carried by the trailer. However, pneumatic suspension systems are typically quite expensive and require additional structural components for providing sufficient roll and wind-up stiffness in most commercial heavy-duty trailer applications.